A "Bag" in NYC, is a glassine envelope, the kind used by stamp collectors. These little wax paper envelopes usually have 100 mgs. Of adulterated heroin (usually 20 to 30% pure), #4 (almost always Colombian for the last several years).
Bags cost $$$ US per, sold individually. You can also buy 10 bags, in what is called a "Bundle." These are usually discounted by 20 to 30% off retail.
10 Bundles make 1 "Brick," which can be further discounted for a total of 50% off retail.
IF you simply cross into New Jersey a "Brick" is no longer 10 Bundles. Instead 5 Bundles make a "Jersey Brick."
IF you move south, to Camden, Atlantic City, and across into Philadeplhia in Pennsylvania, a "Bundle" becomes 13 bags.
When you hit southern Deleware it changes completely to Baltimore standards. In Baltimore they sell glassines, vials and a couple of other ways. They have "Scramble," and "P-Dope" which are supposed to be terms describing quality (P-Dope being purer), etc.
Until Baltimore almost all bags will have a "Stamp" on it, denoting a "Brand." They are usually topical "Bin Laden," or tongue in cheek adverts like, "Dirty Urine" or "DOA."